Explain what we mean by pre-oxidation and pre-reduction. Why is it important to be able to destroy the reagents used for these purposes?

• Before titration, the oxidation state of the analyte may also want to be adjusted.

• Pre-adjustment should be quantitative, and extra pre-adjustment reagent should be eliminated to keep away from interfering with the succeeding titration.

• Peroxydisulfate, Silver (II) oxide, and sodium bismuthate are amazing oxidants that may be without problems removed following pre- oxidation.

• Adjustment of oxidation state before the titration is called pre-oxidation and pre-reduction.

• Pre-oxidation is an increasement of the oxidation number, for example ${\mathrm{Mn}}^{2+}$to ${\mathrm{MnO}}_4‐$.

• Pre-reduction is a decreasement of the oxidation number, for example${\mathrm{Fe}}^{3\mathrm{\iota }}$to ${\mathrm{Fe}}^{2\mathrm{\iota }}$.

• The reagent needs to be destroyed so it doesn't interfere with analytes during the titration.

• Reagents that are used for pre-oxidation and pre-reduction are usually destroyed by boiling.

Pre-oxidation:

Peroxydisulfate $\left({\mathrm{S}}_2{{\mathrm{O}}_3}^{2‐}\right)$- requires $\mathrm{Ag}-$ as a catalyst.

${\mathrm{S}}_2{{\mathrm{O}}_3}^{2‐}+{\mathrm{Ag}}^{+}\to {\mathrm{SO}}_4^{2‐}+{\mathrm{SO}}_4^{‐}+{\mathrm{Ag}}^{2+}$

Excess reagent destroyed:

$2{\mathrm{S}}_2{\mathrm{O}}_8^{2‐}+2{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{boiling}}{\to }4{\mathrm{SO}}_4^{2‐}+{\mathrm{O}}_2+4{\mathrm{H}}^{-}$

Pre-reduction:

Stannous chloride $\left({\mathrm{SnCI}}_2\right)$will reduce ${\mathrm{Fe}}^{3\mathrm{\iota }}\mathrm{to}{\mathrm{F}}^{2\mathrm{\iota }}$in hot

Excess reductant is then destroyed:

${\mathrm{Sn}}^2+2{\mathrm{HgCI}}_2\to {\mathrm{Sn}}^4+{\mathrm{HgCI}}_2+2\mathrm{CI}$